<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script type="text/javascript">
    var  a= 13;
    var  b= 1;
    			for (var i = 1; i <= a; i++) {
                    b *= i;
    			}
    			alert(b);

</script>
</body>
</html>